Coated metallic uranium article and method of making



United States Patent 3,211,628 COATED METALLIC URANIUM ARTICLE AND METHOD OF MAKING Lowell D. Eubank, South Euclid, Ohio, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Mar. 18, 1946, Ser. No. 655,367 9 Claims. '(Cl. 176-82) This invention relates to the protection of metallic uranium surfaces and is particularly concerned with the application of protective metal coatings on uranium metal.

Uranium is a reactive metal which is readily attacked by oxidizing agents and readily corroded by reagents as mild as ordinary water. Consequently in the use of this metal in neutronic reactors, of the type described in the copending application of Fermi et al., Serial No. 568,904, filed December 19, 1944, now U.S. Patent No. 2,708,656, granted May 17, 1955, it is necessary to provide a protective coating for the metal to avoid injury by gases or liquids with which the metal comes in contact in the process. In the past the metallurgical behavior of uranium has not been studied extensively and the development of protective coatings for this metal has provided a new field for study. Early in the investigations of possible protective coatings for metallic uranium it was found that the protective coatings commonly employed in the protection of iron and steel were unsuitable for the protection of uranium in a neutronic reactor either because of the unusually corrosive conditions involved or because of interference of the coating metal with the normal functioning of the reactor.

The present invention has for an object the provision of protective coatings on metallic uranium and as a further object the protection of metallic uranium against the corrosive action of gases or liquids involved in neutronic reactor operations. Further objects will be apparent from the following description of the invention and detailed examples of its application.

In accordance with my invention a metallic uranium article to be protected is provided with an aluminumsilicon alloy coating firmly bonded thereto. Suitable coatings may be applied by dipping the metallic uranium into a molten aluminum silicon alloy of the desired composition. In order to secure the most satisfactory results it is desirable prior to dipping the uranium into the molten metal to prepare the metallic uranium surface so as to remove foreign matter and provide a surface to which maximum adherence can be achieved. The aluminum silicon alloys commonly known as casting alloys are suitable for preparing the coatings of the invention. Aluminum silicon casting alloys are aluminum alloys of silicon in which the aluminum predominates. The principal alloys of this type are those containing to 20% by Weight of silicon and the remainder essentially aluminum. Minor proportions of other metals such as zinc, copper, and nickel may be added to secure improved properties for specific purposes. Alloy improvement processes such as degassing the aluminum silicon alloy with chlorine or a mixture of chlorine and nitrogen to eliminate hydrogen, and modification with small quantities of sodium may be employed to produce coatings of superior properties.

The aluminum silicon coatings may be applied from baths with or without protective flux. Suitable fluxes are alkali-metal chloride fluxes such as potassium chloride, lithium chloride fluxes to which small amounts of fluorides may be added to improve their eifectiveness. A flux containing about 42% lithium chloride, 53% potassium chloride and 5% sodium chloride (the triple eutectic) with or without 5% to 10% of sodium fluoride added to the mixture has been employed very satisfactorily. The

3,211,628 Patented Get. 12, 1965 preparation of metallic uranium surfaces for the application of coatings by means of an alkali-meal chloride or bromide flux is the subject of my U.S. patent application Serial No. 583,176, filed March 16, 1945, now U.S. Patent No. 2,847,321, granted August 12, 1958.

The preferred aluminum silicon alloys are those melting at about 580 C. These alloys contain between 10% and 15% of silicon and may in addition contain from a fraction of a percent to 1% or more of zinc, sodium, nickel or copper. The most suitable application temperatures for such compositions lie between 590 C. and 660 C. The optimum time of dipping depends upon the composition of the coating metal and the temperature of application and normally lies between 15 seconds and seconds. The optimum combination of temperature and time is affected by the pretreatment or preheating of the metal. Thus if the metallic uranium has been heated to a temperature close to the coating temperature by immersion in a preheating bath, the temperature or time in the aluminum silicon alloy may be correspondingly reduced.

The aluminum silicon coatings of the invention may be used alone for the protection of metallic uranium articles or may be used very advantageously for brazing the metallic uranium to an overlying sheath of metallic aluminum. In the latter case the aluminum silicon alloy serves not only to provide a firm metallic bond between the uranium and the aluminum sheath, thus insuring maximum efiiciency of heat transfer, but provides an additional thickness of protective metal for the uranium, thus insuring against failure of the surface protection in the event of flaws or defects in the metallic aluminum sheaths.

While a variety of preparatory treatments are available for preparing the uranium for coating, many of these fail to provide a surface permitting application of adherent coatings of optimum properties. Consequently it is preferred to employ a preparatory treatment such as that described in U.S. patent application Serial No. 619,265, filed September 28, 1945, by Ernest R. Boller, Lowell D. Eubank and John W. Robinson, now U.S. Patent No. 2,848,361, granted August 19, 1958. The preparatory treatment described in this application comprises an immersion or washing of the uranium surface in aqueous nitric acid of 50% to 70% concentration at 60 to 70 C. for 4 to 6 minutes.

Aluminum silicon alloy coatings suitable for protecting uranium from corrosive agents for a considerable time may be prepared by the hot dipping process alone. However where exceptionally heavy coatings are desired, they may be built up by pouring additional aluminum silicon alloy onto rollers and rolling the coating metal onto the hot-dipped uranium as described and claimed in U.S. patent application Serial No. 646,168, filed February 7, 1946, now U.S. Patent No. 2,994,951, granted August 8, 1961.

The following examples further illustrate the invention. In the examples quantities are expressed in terms of weight unless otherwise indicated.

Example 1 A small metallic uranium rod having a machined surface was prepared for coating by pickling for 6 minutes in aqueous 50% nitric acid at 65-70 C., rinsing in water, and drying. The rod was then coated by dipping it through a 42% lithium chloride, 53% potassium chloride, 5% sodium chloride fiux into a molten metal bath comprising 88% aluminum and 12% silicon at 625 C. It was held in this bath for 3 seconds, then removed and placed on Transite rollers where it was rolled slowly for one minute while the coating solidified. The rod was then placed in an annealing oven at 250 C. for about 3 3' hours. The rod was coated on its machined surface by a reasonably smooth continuous layer of aluminum silicon alloy.

Example 2 A small machined metallic uranium rod was prepared for coating by dipping for minutes in 50% nitric acid at a temperature of 6570 C., rinsing and drying. The rod was then dipped through a flux of the composition previously described into a molten aluminum silicon alloy comprising 88% aluminum and 12% silicon at 650 C. for three minutes, removed, and immediately redipped for 40 seconds more. The rod was then placed on Transite roller to cool and permit the coating to solidify. The machined surface of the rod was uniformly coated with a smooth aluminum silicon alloy coating.

Example 3 An extruded uranium rod of the type employed in Example 1 was dipped into an aqueous 50% nitric acid solution at 6070 C. for 5 minutes then removed, rinsed, and dried.

It was then dipped directly into an 88% aluminum, 12% silicon bath. The period of immersion in the aluminum silicon bath was 1 /2 minutes and the temperature was 610 C. The rod was rolled as in Example 1 until the coating solidified; it was then cooled in air to room temperature. A smooth, uniform, firmly adherent coating of aluminum silicon alloy over a hard intermediate uranium-aluminum-silicon alloy layer was formed.

Example 4 A small metallic uranium rod was prepared for coating by treatment for five minutes in aqueous 50% nitric acid at 6070 C., rinsed, and dried. The rod was then immersed directly into a fiuxless 88% aluminum, 12% silicon bath maintained at 650 C. for 3 minutes. It was then withdrawn from the bath and cooled by rolling in air on Transite rollers. A smooth, continuous coating of the aluminum silicon alloy was obtained on the rolled surface.

Example 5 A number of metallic uranium rods prepared for coating as described in the preceding examples was canned employing aluminum cans and aluminum silicon alloy as a bonding medium for brazing the rods to the cans. In each case the metallic uranium rod, without preheating, was dipped into a fiuxless molten metal bath consisting of fresh aluminum silicon alloy (Federal Specification AN-QQ-A-366, Amend. 4A1-l3X) at a temperature of 615 C. and maintained therein for 30 seconds; the rod was withdrawn from this bath and placed directly in the can which was then cooled. The cans were 28 aluminum cans having an inside diameter mils greater than the diameter of the rod. They were precleaned by washing first with trichlorethene, then with an aqueous solution containing 0.1% soap and 0.1% sodium pyrophosphate at 80 C., and finally with ortho-phosphoric acid solution for five minutes at 20 C. Each can was held at 640 C. in a steel supporting sleeve and 80 grams of aluminum silicon alloy was added just before the uranium rod was placed in the can. This amount of the alloy was substantially in excess of that required to fill the space between the can and the rod. By radiographic tests the average proportion of the area of voids in the bonding layer was found to be 0.1%, indicating a good bond between the metallic uranium and the aluminum sheath.

Example 6 A metallic uranium rod prepared as in Example 1 was dipped through a potassium lithium sodium chloride flux into a molten bath of 88% aluminum, 12% silicon alloy at a temperature of about 630 C. for two minutes. The rod was removed and redipped for one minute in a similar bath at a temperature of about 625 C. Upon removal 4 from this bath the rod was cooled on Transite rolls and annealed for three hours at 250 C. An adherent continuous protective coating was obtained.

Example 7 The procedure was the same as employed in the preceding example except that a single dip in the aluminum silicon alloy was effected with the alloy at a temperature of 650 C. for about 45 seconds. A firmly adherent, continuous protective coating was obtained in this case also.

It will be understood that I intend to include variations and modifications of the invention and that the preceding examples are illustrations only and in no wise to be construed as limitations upon the invention, the scope of which is defined in the appended claims, wherein I claim:

1. A metallic uranium article having an aluminum sheath bonded to the uranium article by an intermediate bonding alloy of aluminum and silicon, said alloy consisting substantially of from 5 to 20% by weight of silicon and from 95 to by weight of aluminum.

2. A metallic uranium article having an aluminum sheath bonded to the uranium article by an intermediate bonding alloy of aluminum and silicon comprising about 10% to 15% silicon and to 85% aluminum in the intermediate layer.

3. The method of protectively coating a metallic uranium article, which comprises dipping the article in a molten bath of aluminum silicon alloy to form an adherent layer of the alloy on the article and cooling the article to cause the layer to solidify thereon, said alloy consisting substantially of from 5 to 20% by weight of silicon and from to 80% by weight of aluminum.

4. The method of protectively coating a metallic uranium article, which comprises dipping the article in an aluminum silicon alloy comprising about 10% to 15% silicon and the balance essentially aluminum at a temperature between 590 C. and 660 C. to form an adherent layer of the aluminum silicon alloy on the metal article and cooling the article to cause the aluminum silicon alloy to solidify thereon.

5. The method of applying a protective coating to a metallic uranium article, which comprises dipping the article in a molten aluminum silicon alloy containing from about 5 to 20% of silicon and the remainder consisting substantially of aluminum, said alloy having a melting temperature of about 580 C., placing the coated article in an aluminum can with sufficient additional molten aluminum silicon alloy to fill the space between the article and the can, and cooling the resulting assembly to cause the aluminum silicon alloy to solidify and thereby bond the aluminum can to the uranium article.

6. The method of providing a protective sheath on a metallic uranium article, which comprises immersing the article in an aluminum silicon alloy comprising about 10% to about 15% silicon and the balance essentially aluminum at a temperature between 590 C. and 660 C. so as to provide the uranium article with an adherent coating of the aluminum-silicon alloy, placing the coated uranium article in an aluminum can containing sufficient molten aluminum silicon alloy to fill the space between the article and the can, and cooling the resulting assembly to cause the aluminum silicon alloy to solidify and thereby bond the aluminum can to the uranium article.

'7. A metallic uranium article having an aluminum sheath bonded thereto by an intermediate alloy containing approximately 12% silicon and the remainder consisting substantially of aluminum.

8. The method of providing a protective sheath on a metallic uranium article, which comprises immersing the article in an aluminum-silicon alloy comprising about 12% silicon and the balance being essentially aluminum at a temperature between 590 and 660 C. so as to provide the uranium article with an adherent coating of the aluminum-silicon alloy, placing the coated uranium article in an aluminum can containing sufiicient of said molten aluminum-silicon alloy to fill the space between the article and the can, and cooling the resulting assembly to cause the aluminum-silicon alloy to solidify and thereby to bond the aluminum can to the uranium article.

9. The method of protectively coating a metallic uranium article, which comprises immersing the article in an aluminum-silicon alloy consisting substantially of about 12% silicon and 88% aluminum and at a temperature of from 590 to 660 C. whereby a layer of said alloy is formed on the uranium article, and rolling the thus coated article while allowing the coating to cool and solidify.

References Cited by the Examiner UNITED STATES PATENTS CARL D. QUARFORTH, Primary Examiner.

WILLIAM G. WILES, JAMES L. BREWRINK, SAM- UEL COCKERILL, Examiners.

UNITED STATES PATENT OFFICE 7 CERTIFICATE OF CORRECTION Patent No 3 211 628 October 12, 1965 Lowell D. Eubank It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 2, for "alkali-meal" read alkalimetal line 41, for "2,848,361" read 2,848,351

Signed and sealed this 27th day of September 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER \ttesting Offioer Commissioner of Patents 

1. A METALLIC URANIUM ARTICLE HAVING AN ALUMINUM SHEATH BONDED TO THE URANIUM ARTICLE BY AN INTERMEDIATE BONDING ALLOY OF ALUMINUM AND SILICON, SAID ALLOY CONSISTING SUBSTANTIALLY OF FROM 5 TO 20% BY WEIGHT OF SILICON AND FROM 95 TO 80% BY WEIGHT OF ALUMINUM.
 9. THE METHOD OF PROTECTIVELY COATING A METALLIC URANIUM ARTICLE, WHICH COMPRISES IMMERSING THE ARTICLE IN AN ALUMINUM-SILICON ALLOY CONSISTING SUBSTANTIALLY OF ABOUT 12% SILICON AND 88% ALUMINUM AND AT A TEMPERATURE OF FROM 590* TO 660* C. WHEREBY A LAYER OF SAID ALLOY IS FORMED ONTHE URANIUM ARTICLE, AND ROLLING THE THUS COATED ARTICLE WHILE ALLOWING THE COATING TO COOL AND SOLIDIFY. 